Re: [PATCH v9 1/8] mm: Introduce memfd_restricted system call to create restricted user memory

Kirill A. Shutemov
2022-11-14T15:29:21Z

On Mon, Nov 14, 2022 at 03:02:37PM +0100, Vlastimil Babka wrote:
> On 11/1/22 16:19, Michael Roth wrote:
> > On Tue, Nov 01, 2022 at 07:37:29PM +0800, Chao Peng wrote:
> >> >
> >> > 1) restoring kernel directmap:
> >> >
> >> > Currently SNP (and I believe TDX) need to either split or remove kernel
> >> > direct mappings for restricted PFNs, since there is no guarantee that
> >> > other PFNs within a 2MB range won't be used for non-restricted
> >> > (which will cause an RMP #PF in the case of SNP since the 2MB
> >> > mapping overlaps with guest-owned pages)
> >>
> >> Has the splitting and restoring been a well-discussed direction? I'm
> >> just curious whether there is other options to solve this issue.
> >
> > For SNP it's been discussed for quite some time, and either splitting or
> > removing private entries from directmap are the well-discussed way I'm
> > aware of to avoid RMP violations due to some other kernel process using
> > a 2MB mapping to access shared memory if there are private pages that
> > happen to be within that range.
> >
> > In both cases the issue of how to restore directmap as 2M becomes a
> > problem.
> >
> > I was also under the impression TDX had similar requirements. If so,
> > do you know what the plan is for handling this for TDX?
> >
> > There are also 2 potential alternatives I'm aware of, but these haven't
> > been discussed in much detail AFAIK:
> >
> > a) Ensure confidential guests are backed by 2MB pages. shmem has a way to
> > request 2MB THP pages, but I'm not sure how reliably we can guarantee
> > that enough THPs are available, so if we went that route we'd probably
> > be better off requiring the use of hugetlbfs as the backing store. But
> > obviously that's a bit limiting and it would be nice to have the option
> > of using normal pages as well. One nice thing with invalidation
> > scheme proposed here is that this would "Just Work" if implement
> > hugetlbfs support, so an admin that doesn't want any directmap
> > splitting has this option available, otherwise it's done as a
> > best-effort.
> >
> > b) Implement general support for restoring directmap as 2M even when
> > subpages might be in use by other kernel threads. This would be the
> > most flexible approach since it requires no special handling during
> > invalidations, but I think it's only possible if all the CPA
> > attributes for the 2M range are the same at the time the mapping is
> > restored/unsplit, so some potential locking issues there and still
> > chance for splitting directmap over time.
>
> I've been hoping that
>
> c) using a mechanism such as [1] [2] where the goal is to group together
> these small allocations that need to increase directmap granularity so
> maximum number of large mappings are preserved.

As I mentioned in the other thread the restricted memfd can be backed by
secretmem instead of plain memfd. It already handles directmap with care.

But I don't think it has to be part of initial restricted memfd
implementation. It is SEV-specific requirement and AMD folks can extend
implementation as needed later.

--
Kiryl Shutsemau / Kirill A. Shutemov